Surface-active agents constitute an extremely important class of industrial chemicals which have a wide variety of uses, for instance as detergents for washing purposes, as emulsifiers in food products and as active ingredients in various personal care products such as shampoos, soaps or creams.
At the molecular level, surface-active agents are substances which are characterized by the presence of hydrophobic and hydrophilic regions within each individual surfactant molecule and which owe their ability to reduce surface tension to this particular structure. The combination of hydrophobic and hydrophilic regions within the same molecule may be obtained in many different ways, for instance by combining a sulphonic acid residue, a quaternized ammonium moiety or a glycerol moiety with an alkyl chain as is the case with the linear alkyl surfactants, the quarternized alkyl amines or the monoglycerides, respectively. When designing a surfactant molecule, the detailed molecular architecture of the compounds is a major concern, care being taken to achieve a precise balance between the hydrophobic and hydrophilic regions of the surfactant molecules as well as to achieve a favorable spatial arrangement of these individual regions of the molecules. Apart from this, the possibility of producing surface-active agents by high-yielding processes and on the basis of inexpensive and readily available raw materials is always carefully considered The environmental issues related to the eventual loading of the surfactant into the environment are finally a matter of major concern.
As a result of these considerations, many researchers have shown considerable interest in the production of surface-active agents based on sugars and fatty acids, e.g. sugar esters. Such substances were expected to exhibit surface-active properties due to the hydrophilic properties of the sugar moieties and the hydrophobic properties of the fatty acid residues The balance between hydrophobic and hydrophilic properties might be varied by modifying the sugar and/or the fatty acid by adding a number of substituents. Such surface-active agents could be produced from very inexpensive starting materials and, being prepared from and degradable into naturally occurring components, they would not constitute an environmental hazard.
One traditional method of preparing sugar esters, including glycoside esters, has been by transesterification. Thus, U.S. Pat. No. 3,597,417 discloses the preparation of alkyl monoglycoside esters by transesterification in a two-step process by reacting a glycoside with a short-chain ester and subsequently with a fatty acid ester. Another method is disclosed in U.S. Pat. No. 2,759,922 in which a process for producing esterified glycosides, e.g. methyl glycoside, by reacting the glycoside with a fatty acid at a temperature of 160.degree.-300.degree. C.
In spite of the intensive interest in producing sugar esters of fatty acids, it has been found rather difficult to produce surface-active sugar esters by conventional synthesis procedures Among other things, this is due to the presence of several chemically similar groups in the sugar molecules which may therefore be esterified at many different positions and to varying degrees when exposed to esterification reagents. Sugar esters prepared by traditional chemical synthesis are therefore inhomogeneous in that they are composed of mixtures of compounds different in the degree of esterification and in the position of the acyl groups on the sugar moiety. This may cause differences in the surface-active properties of the compounds. As, additionally, the preparation of sugar esters by conventional chemical synthesis has been found to be rather cost-intensive, the currently available sugar esters prepared by these methods have found limited application only.
In view of the difficulties encountered in the production of sugar esters by chemical synthesis and in view of the attractiveness of these compounds as surface-active agents, alternative methods have been suggested for the production of esterified sugars, one interesting method involving the use of enzymes which are known to be highly regioselective and enantioselective so that they may be employed for the selective esterification of one or more hydroxy groups on the sugar molecules. Such enzymatic processes may exploit cheap starting materials which means that the resulting sugar esters are inexpensive even though they are of a high quality.
The attempts to develop efficient enzymatic syntheses of sugar esters have so far not been particularly successful. Thus, Sweers and Wong (J. Amer. Chem. Soc. 108, 1986, pp. 6421-6422) briefly discuss the regioselective esterification of sugars, e.g. methyl glycoside, with pentanoic acid in the presence of a Candida cylindracea lipase and report that the yield of this process was very low (2-3%). Similarly, U.S. Pat. No. 4,614,718 discloses the preparation of sugar or sugar alcohol esters by reacting the sugar or sugar alcohol with a higher fatty acid in finely divided or emulsified form in the presence of lipase until an equilibrium is obtained. A large amount of water is used as solvent and as a result of this, the equilibrium of the reaction cannot be shifted which means that the yield cannot be optimized Furthermore, the reaction proceeds for a considerable length of time even though large amounts of the enzyme are employed.
One reason why poor yields are obtained and/or long reaction times are required in the known enzymatic processes is the considerable difference in polarity between the sugar component and the fatty acid component which makes it difficult to find a solvent in which both are soluble. When using water as a solvent as taught in U.S. Pat. No. 4,614,718, the fatty acid is not dissolved resulting in an inefficient reaction and a low utilization of the fatty acid reagent. Few solvents for both sugars and fatty acids are available (e.g. dimethylformamide) and such solvents will generally inactivate the enzyme and are in most cases toxic, constituting an environmental hazard.
JP 62-195 292 discloses a method of preparing sugar or sugar alcohol esters by reacting a sugar or sugar alcohol with a fatty acid in an aqueous medium in the presence of a lipase after which the water is gradually removed and incubation is continued. JP 62-289 190 discloses a method of preparing sugar or sugar alcohol esters by mixing sugar or sugar alcohol, fatty acid and lipase and adding only a minor amount of water to the reaction mixture JP 63-112 993 discloses a method of preparing sugar or sugar alcohol esters by reacting an acetylated sugar or sugar alcohol with a fatty acid in an organic solvent in the presence of a lipase.
An object of the present invention is to provide a process for the production of methyl glycoside esters in high yields from inexpensive materials by enzymatic catalysis without the use of toxic solvents.
Another object of the invention is to provide methyl glycoside esters which are particularly useful as surface-active agents in cleaning compositions and personal care products.